24" intel iMac shuts off during startup

Every time I switch on my iMac, the Apple logo comes on for a couple of seconds then the computer switches itself off.

It's a 2008 model, 24" screen, aluminum body.

I've already tried loading the original Apple Install Disc (pressing C) but it won't load. I've also tried resetting the PRAM, didn't do anything either.

What do you think could be the cause? Is there anything else I could do? Do you think it's worth taking it to apple for repair?

Regarding the contents of the hard drive, is it possible to retrieve data from it using the Target disc mode (I also have a MacBook) even though my iMac is dead? Not sure if that's possible...?

Sounds like a major hardware failure. Off to Apple.

It won't hurt to TRY using Target disk mode:
How to use and troubleshoot FireWire target disk mode

Did you also try and reset the PMU (Power Management Unit)? My 20" same as your 24" spec wise did that and resetting the PMU fixed it.

[HOWTO] Reset the System Management Controller (SMC) - macosx.com

Thanks for the advice guys! I tried to reset the PMU, that didn't work.... BUT I was able to access the hard drive in Target disk mode and was able to recover all of its contents! I'm glad I tried that!!

I will take the iMac to Apple tomorrow, and will post more info when I hear again from them.

The hard drive had to be changed. 199€ labor included. They're putting in a 500Go instead of the old 300Go. Thanks to everyone for the help.